More Scheme

CS 331

Quiz 1. What is (car ‘((2) 3 4))?

(2)

2. What is (cdr ‘((2) (3) (4)))?((3)(4))

3. What is (cons ‘2 ‘(2 3 4))?(2 2 3 4)

4. What is the length of the list ‘(()()()())?4

5. Which element does (car (cdr ‘(x y z))) extract from the list?y

6. What do some people thing LISP stands for?Losta Insane Stupid Parenthesis

Reading Input

• (read) returns whatever is input at the keyboard

• (define x (read))

55

x

55

Display Output

• Use display:

• (display (+ 3 4))• (display x)• (newline) ; output newline

• Can use to add to functions for debugging or use Trace functions

Trace – Shows function calls

• Choose “Pretty Big or R5RS)” as your language• Add (require (lib "trace.ss")) to the top of your

program• Add (trace functionname) or (untrace

functionname) to turn off tracing

Structure of Lists

• List : a sequence of zero of more elements• May be heterogeneous

– (a 20 (20 20) (lambda (x) (+ x x)))

• List of Lists• (+ 3 4)

– A list

– An expression

• Allows expression to be built and then evaluated

Tree structure of lists(a b c)

a

b

c ( )

(cons a

(cons b

(cons c ‘())

)

)

List structure

(a (b) (c (d)))

a

( )b ( )

c

d ( )

List storage

• Independent of allocation schemes

• Familiarity is helpful for assessment

• Cons is the constructor– Allocates a single cell

head

tail

List structure

(a (b) (c (d)))

( )

a

b ( ) c d ( )

Notion of Equality

• Eq? – Checks if the two pointers are the same

• Equal? – Checks if the two arguments are lists with

“equal” elements.– Recursive– Structurally the same

• Eq? equal? for symbols.

Examples

(equal? ‘foo ‘foo) (eq? ‘foo ‘foo) true

(equal? ‘(a b) ‘(a b)) true

(eq? ‘(a b) ‘(a b)) false

(define x ‘(a b c))(define y (cons (car x) (cdr x)))(equal? x y)

true(eq? x y)

false

(define mylist ‘(a b c))(eq? (member ‘b mylist) (cdr mylist))

true

More List Functions

• append takes two arguments and returns the concatenation of two lists. be careful here not to confuse append with cons.

(append '(a b c)  '(d e f))  (a b c d e f) (append '(a b (c))  '((d) e f))   (a b (c) (d) e f)

• list returns a list constructed from its arguments.  (list 'a) (a) (list 'a 'b 'c 'd 'e 'f) (a b c d e f) (list '(a b c)) ((a b c)) (list '(a b c) '(d e f) '(g h i)) ((a b c)(d e f)(g h i))

More List Functions

• length returns the length of a list. (length '(a b c d ef gh i jk)) 8 (length '(a b c d (ef gh i jk))) 5

• reverse returns the same list, only in reversed order. Note, this is only shallow reverse.

(reverse '(a b d g o)) (o g d b a) (reverse '(a (b d) g o))   (o g (b d) a)

Exercise

• Write a helper function to return the first half of a list. Here is the main function:

• Write a helper function to return the second half of a list. Here is the main function:

(define (firsthalf lst) (getfirsthalf lst (quotient (length lst) 2)))

(define (secondhalf lst) (getsecondhalf lst (quotient (length lst) 2)))

Merge Method

• Write a method called Merge that merges two sorted lists:(define (Merge x y)

)

MergeSort

• Write a MergeSort method that uses your Merge method to sort a list of numbers.

let & let*

(let ((x1 E1)

(x2 E2)

(x3 E3)

….

(xn En))

F)

- Expressions E1, E2, … En are evaluated

- Evaluate F with xi’s bound to Ei’s

- Value of let is the value of F

Local Variables(let & let*)

• Used to factor out common expressions

• Introduce names in subexpressions

• Order of evaluation of expression is undetermined (let)

• Order of evaluation of expression is sequential (let*)

Examples

(let ((x 2)

(y x))

y)

0

(let* ((x 2)

(y x))

y)

2

Tail recursion

A recursive function is tail-recursive if

(a) it returns a value without needing recursion OR

(b) simply the result of a recursive activation

i.e. just return the value at the end

• Can be efficiently implemented– Don’t need stacks

• Can convert many functions to be tail recursive

Examples

(define factorial

(lambda(n)

(cond ((= n 1) 1)

(else (* n (factorial (- n 1)))))))

Factorial Function

ExamplesFactorial Function: Tail Recursive

(define factorial2

(lambda(n m)

(cond ((= n 1) m)

(else (factorial2

(- n 1)

(* m n))))))

(define factorial

(lambda(n)

(factorial2 n 1)))

Examples

Getridof Function (get rid of an item from a list)

(define getridof

(lambda(list item)

(cond ((null? list) '())

((equal? item (car list)) (getridof (cdr list) item))

(else (cons (car list) (getridof (cdr list) item))))))

Examplesgetridof Function (Tail recursive)

In reverse order! Could you put in original order?

(define gro

(lambda(list item list2)

(cond ((null? list) list2)

((equal? item (car list)) (gro (cdr list) item list2))

(else (gro (cdr list) item (cons (car list) list2))))))

(gro '(x y x z x w) 'x '())

(w z y)

Functions

• Functions are first class citizens in Scheme– Variables may be bound to functions

– Can be passed as parameters

– Can be returned as values of functions

Functions as First Class Citizens

A function may be bound to a variable, we are already doing this:

(define add1 (lambda(x) (+ 1 x)))

(add1 4)

5

Functions as First Class Citizens

Functions can be passed as parameters

(define (foo x y) (x y)

)

(foo (lambda(x) (+ 1 x)) 4)5

Functions as First Class Citizens

Functions can be returned as values of functions

(define (foo x) (lambda(y) (+ x y)))

(foo 4)

#<procedure:2:16>

((foo 4) 5)9

Examples(map)

(map cdr ‘((1 2) (3 4)) ((2) (4))(map car '((a b) (c d) (e f) (g h)))(a c e g)

• Takes two arguments– Function and a list– Applies function to the list

Examples(map)

(let

((proc (lambda(ls) (cons 'a ls))))

(map proc '((b c) (d e) (f g h))))

((a b c) (a d e) (a f g h))

Apply

(apply + ‘(4 11))

15

(apply max ‘(3 4 5))

5

(apply <function> <arguments-in-a-list>)

Useful when the arguments are built separately from application

Eval

• Eval will evaluate the parameter as a valid Scheme expression

(eval ‘(car ‘(a b c))a

(eval ‘(define (foo a b) (+ a b)))foo

(foo 3 4)7

Allows for interesting opportunities for code to modify itself andexecute self-generated code

Binding of Variables

• Global binding– define

• Local binding– lambda, let

• How do we change the value of a variable to which it is bound?– We have been using define multiple times, although in

some implementations of Scheme this is invalid

set!

(set! var val)

• Evaluate val and bind it to var• ! Indicates a side effect

• Scheme does not specify what this returns– Implementation dependent

– DrScheme seems to return nothing

Examples

(define f (lambda(x) (+ x 10)))

(f 5)

>15

(set! f (lambda(x) (* x 10)))

(f 5)

>50

Examples

(let ((f (lambda(x) (+ x 100)))) (display (f 5)) (newline) (set! f (lambda(x) (* x 100))) (f 5))

105 500(f 5) Error, reference to undefined identifier: f

Scheme uses lexical scoping.

set-car! & set-cdr!

(define x '(4 5 6))

(set-car! x 7)

x

> (7 5 6)

(set-cdr! x '( 7 8 9))

x

>(7 7 8 9)

More Examples

(define (my-reverse ls)

(cond ((null? ls) '())

(else (append (my-reverse (cdr ls))

(list (car ls))))))

(my-reverse '(a b c d))

Recursive Functions

(define (atom? x) (not (list? x)))

(define (super-reverse ls)

(cond ((null? ls) '())

((atom? ls) ls)

(else (append (super-reverse (cdr ls))

(list (super-reverse (car ls)))))))

(super-reverse '((a b) ((c d) e) f))

Recursive Functions

(define (pairup x y) (cond ((null? x) '()) ((null? y) '()) (else (cons (list (car x) (car y)) (pairup (cdr x) (cdr y))))))

(pairup '(a b c) '(1 2 3))

Recursive Functions

(define (listify ls)

(cond ((null? ls) '())

(else (cons (list (car ls))

(listify (cdr ls))))))

(listify '(1 2 2 3))

Summary

• Pure functional programming• No assignments (side effects)• Refreshingly simple • Surprisingly powerful

– Recursion- Functions as first class objects

• Implicit storage management– Garbage Collection